Organic matrix composites are perhaps the most widespread composite materials. Two types of polymer resins can be used for organic matrix composites: thermosets and thermoplastics. Also a wide range of fibers, such as carbon, aramid, vegetal, and glass fibers can be used to reinforce the organic matrix of the composite material. In this respect, glass fibers are widely used.
Sizing compositions are traditionally used in the production of glass fibers to increase the manufacturing, transformation, and general processing characteristics of the glass fibers. The use of sizing compositions may further improve the performance of glass and other fibers in composite materials by increasing the compatibility of the fibers with the matrix material used in forming the composites.
Sizing compositions are thin coatings that are applied to glass fibers as the fibers are being formed. In this respect, sizing compositions differ from secondary or post-coating compositions that are applied in a process separated from the glass fiber production process. Such secondary or post-coating compositions are typically much thicker (e.g., 1,000 times n glass fiber sizings and typically have a thickness in the range of 50-200 micrometers. In contrast, the typical sizing thickness is around 50 nm and generally represents less than 1 weight % of the glass fiber.
Traditionally, the sizing compositions used to coat glass fibers are aqueous-based compositions, as either a suspension or emulsion. Such aqueous sizing compositions conventionally contain up to 90 weight % water and 10 weight % dry matter. The suspension or emulsion has a solids content that is often composed of at least a film former, a coupling agent, a lubricant, and a surfactant.
A film former may be used in a sizing composition to hold individual filaments together to form fibers, and protect the filaments from damage caused by abrasion. Traditional film formers include polyvinyl acetates, polyurethanes, modified polyolefins, polyesters epoxides, and mixtures thereof, with various molecular weights from 600 g/mol to more than 20,000 g/mol.
Sizing compositions may also include a coupling agent to enhance the adhesion of the sizing compositions with matrix material when forming a composite, to improve the composite properties. A suitable coupling agent can be an organofunctional silane.
Additional additives may be included in the sizing compositions, depending on the intended application. Such additives include, for example, anti-statics, wetting agents, antioxidants, and pH modifiers.
However, such aqueous sizing compositions have certain drawbacks. For example, once glass fibers are sized, the sized glass fibers are gathered into a strand and the strand is collected on a forming package. Prior to packaging, he glass strand must be completely dried to remove the water from the sizing composition, leaving only solids on the glass strands. To accomplish this, the glass strands are subjected to a specific temperature cycle, with temperatures up to about 150° C. The drying may take up to 24 hours and specific drying equipment is required, which requires additional capital and energy expense.
Another drawback is a phenomenon called migration, which occurs during the aforementioned drying and packaging process. Migration is the term given to the outward advance of the sizing composition from the glass fibers in the package when the package is dried. The heat from the oven causes the moisture from the sizing composition to be driven outwardly. When this occurs, some of the other sizing ingredients are carried therewith and are deposited on the outer surfaces of the forming package. This requires a stripping process to remove external migration and improve homogeneity of the glass fiber package. However, this supplementary process creates a substantial amount of waste product and further capital expense.
To resolve this issue, UV-curable sizing compositions have been proposed that include free radical photo polymerization using specific monomolecular or bimolecular photo initiating systems. Acrylate and methacrylate-based systems represent the conventional UV-curable compounds for their high reactivity. In this respect, EP 570283 A1 describes an acrylate-based system, in which a liquid UV-curable mixture is deposited on glass filaments directly after they are formed and yet before the winding step.
However, conventional UV-curable sizing compositions experience an oxygen-induced inhibition of radical polymerization. This inhibition is caused by the presence of dissolved oxygen, which establishes an induction period, reducing the polymerization rate and decreasing the final conversion. The reduction of the polymerization rate also reduces the overall polymer length and forms tacky surfaces. Glass fiber sizing compositions with a typical thickness in the nanometer-range are particularly exposed to this phenomenon.
To resolve oxygen interaction in UV-curable glass fiber sizing compositions, physical (e.g., adjusting curing conditions) and chemical solutions (e.g., additives to interact with oxygen or peroxyl radicals to regenerate initiating radicals) have been proposed.
In the field of adhesives, thiol-ene systems have been proposed to avoid inhibition by oxygen in radical based UV curing systems. In this respect, US 2005/0119366 proposes UV-curable adhesive composition comprising a vinyl-ether terminated urethane and a poly-functional mercaptan.
Thiol-ene chemistry describes the reaction of a mercaptan (RSH, thioalcohol, thiol) functional compound with various classes of unsaturated organic compounds (“ene”). When suitable reactants are combined and exposed to an appropriate UV source, the thiol-ene reaction proceeds rapidly and quantitatively, in the presence of ambient oxygen and in the absence of added photoinitiator.
However, no glass fiber sizing system has yet been proposed that avoids both the disadvantages of aqueous glass fiber sizing systems and of the conventional UV-curable glass fiber sizing compositions.